An electrophotographic photosensitive member using an organic photoconductive substance (organic electrophotographic photosensitive member) has the advantages over an electrophotographic photosensitive member which uses an inorganic photoconductive substance (inorganic electrophotographic photosensitive member) of being easier to produce, and having a higher degree of freedom in functional design because the materials for the organic electrophotographic photosensitive member can be selected from a large variety of materials. With the rapid spread of laser beam printers in recent years, such an organic electrophotographic photosensitive member has come to be widely used in the market.
A typical electrophotographic photosensitive member has a support and a photosensitive layer formed on the support. In addition, a laminated photosensitive layer formed by superimposing in order from the support side a charge-generating layer containing a charge-generating substance and a hole-transporting layer containing a hole-transporting substance is often used as a photosensitive layer.
In addition, an intermediate layer is often provided between the support and the photosensitive layer for the purpose of, for example, covering defects on the surface of the support, improving adhesion between the support and the photosensitive layer, suppressing an interference fringe, protecting the photosensitive layer from electrical breakdown, and inhibiting holes from being injected from the support into the photosensitive layer.
Although such an intermediate layer has the above-mentioned merits, it also has the drawback that charge tends to accumulate in the intermediate layer. When image formation is repeatedly performed for a long time period, the accumulation of charge in the intermediate layer can increase potential variation, which can cause problems in an output image.
Japanese Patent Application Laid-Open No. 2005-221923 and No. 2007-148357 disclose a technique for alleviating potential variation or suppressing interference fringe by incorporating surface-treated titanium oxide particles which have a small particle size into an intermediate layer. However, there is still room for improvement in terms of potential variation when image formation is repeatedly performed for a long time period.
In addition, Japanese Patent Application Laid-Open No. S59-84257, No. H09-90661, and No. 2000-66432 disclose a technique for reducing potential variation such as an increase in residual potential or a reduction in initial potential when image formation is repeatedly performed by using an electrophotographic photosensitive member having an intermediate layer. Under the existing circumstances, deterioration in initial sensitivity or deterioration in chargeability may occur, and thus there are still problems which have not been sufficiently solved.
With the increased speed, improved image quality, and trend towards full color of electrophotographic apparatuses in recent years, a problem has arisen in that when image formation is repeatedly performed, potential variation (variation in dark potential (charge potential) or light potential) is suppressed to a greater extent. Specific examples of potential variation include the following.    (1) Potential variation over a relatively long-term (a time period from when the electrophotographic photosensitive member starts to be used until the electrophotographic photosensitive member reaches the end of its life).    (2) Potential variation over a relatively short-term (for example, a time period from the first sheet until about 1,000 sheets in continuous image formation).
There is a need to suppress such potential variation to a greater extent.
Concerning the above item (1), in general, the longer the time period for which the electrophotographic photosensitive member is used, the larger the deterioration in the potential characteristic of the electrophotographic photosensitive member is. Even when the electrophotographic photosensitive member which has already been used for a long time period is left to stand, a possibility is low that the potential characteristic returns to the state at the time of the initiation of the use of the electrophotographic photosensitive member. Accordingly, it can be said that the recoverability of the long-term potential variation described in the above item (1) is insufficient.
Concerning the above item (2), for example, although the electrophotographic photosensitive member rotates several times for forming an image on an A4 size sheet of paper, the potential characteristic of the electrophotographic photosensitive member fluctuates in the sheet, and hence the tint or density of an output image may change. In addition, when outputting the same image on multiple sheets, the density of the image may be different between the first sheet and the n-th sheet (where n>1). Such a short-term potential variation becomes prominent when image formation is performed under a low-humidity environment.
Such short-term potential variation recovers to some extent by leaving the electrophotographic photosensitive member to stand after the use of the electrophotographic photosensitive member.
The long-term potential variation described in the above item (1), which has insufficient recoverability, is thought to be caused by gradual accumulation of variations which are left unrestored in the electrophotographic photosensitive member from repeated use as described in the above item (2).
The electrophotographic photosensitive member should be able to perform image formation stably at all times while suppressing both the long-term potential variation described in the above item (1) and the short-term potential variation described in the above item (2).